An acid test means a decisive test that proves the real quality of something. This answer is correct because the expression has a fixed figurative meaning in English. In an SSC idiom question, students should match the whole phrase with its accepted meaning rather than translate each word separately.

• Classroom example: An acid test can refer to a decisive test that proves the real quality of something.
• Story example: A writer may use An acid test when describing a decisive test that proves the real quality of something.
• Exam example: When a sentence contains An acid test, choose the meaning “a decisive test that proves the real quality of something.”